1. Field of the Invention
The present invention relates to the wired and wireless video communication systems, and a method for controlling the target bit error rate of each packet in wired and wireless video communication systems which support efficiently a video service by controlling the target BER differently in each packet unit. Also, the present invention relates to a video transmission method in wired and wireless video communication systems, which adopts the said method for controlling the target BER. Voice and data services have been mainly served in communication service systems until present, but it is getting certain that video service will be a very important communication service system in the coming wired and wireless video communication systems like IMT-2000, etc.
2. Description of the Related Art
Most video coder such as MPEG, H.261, H.263 compress the video data using techniques such as discrete cosine transform (DCT), motion compensation (MC), variable length coding (VLC). Wherein, the representative example of the VLC is the Huffman code that allocates small number of bits to the often-occurring pattern in rate, and allocates many number of bits to the less-often-occurring pattern in rate, then reduce the total number of bits. However, resistance against the error becomes lower in case of compressing video data too much. In other words, if error occurs in a certain part of a video, a video decoder can't get the sync of the variable length code, and until the sync is acquired, the video data is got damaged. Additionally, the video distortion within a frame occurred by a channel error resulted is propagated into the further frames because the motion compensation is performed. Wherein, the distortion is defined as the mean square error (MSE) of difference between the originally inputted video into encoder and actually reconstructed in the decoder.
To get over the distortion in the video data due to the said channel error, many error concealment methods are suggested. The error concealment method reconstructs the damaged video area part using the receiving video information only. If this said method is used, although the original video can't be reconstructed perfectly, there is an advantage that can reconstruct the video distortion, which is occurred by the uncorrectable error with channel coding, in real time without increasing of transmission rate.
The error concealment method can be roughly grouped in two parts, a temporal concealment method and a spatial concealment one. In the spatial method, the video is reconstructed using a method like interpolation by using the surrounding video information. With using this method, there is an advantage that it is possible to reconstruct clean outlines of objects. However, there is a disadvantage that because it needs lots of calculating to embody, the complication of hardware is getting increased, and if the surrounding video information is damaged, a capability goes down a lot.
However, in the temporal concealment method, the damaged video area due to using motion vector is replaced with the motion compensated former frame area.
This method has an advantage that if there is no big motion change in the video, or if it is not a case which the video is composed too complicatedly, it shows a good capability. Also, it can be embodied comparative simply. The motion vector can be transmitted separately through a channel having comparatively low error occurring rate. In case not using the said channel, the motion vector of the damaged area can be assumed from the motion vector of the area without errors around the damaged video area. When all of the methods are impossible, zero vector can be used as a motion vector.
When encoded video data passes through the wired and wireless channels and is transmitted, a forward error correction (FEC) method is used in the actual communication system. The FEC method adds particular information on the transmission data and then corrects the errors by using the mentioned information at a receiving end when errors occur. In the conventional video communication systems, the same target of BER is given for all the packets of same kind of services such as telephone, conference through video communication network, and synchronized video service, etc. To satisfy this target of BER, in the conventional method, if other conditions are the same such as channel environment, etc., the FEC method is performed by using the same parameter of the FEC. In other words, the convolution coding and the turbo coding use the same code rate and the constraint length, and the BCH code and the lead Solomon code add the same amount of channel coding bit for each packet. After all, when the channel error rate passes through a constant channel, the BERs in the each packet become the same as all the video packets of the same kinds of services.
On the other hand, it is getting certain that for the 3rd generation mobile radio communication systems, the code division multiple access (CDMA) technique will be used. In the CDMA method, user's own code is allocated respectively to communicate. Many users use the same frequency band at the same time. So, the capacity of CDMA system is limited by the entire transmitted power which is received at or transmitted from the base center. Therefore, controlling the transmitted power in the CDMA system is very important and the lots of studies for it have been done. For example, in case of sending the data to the AWGN channel by using BPSK modulation, BER is given as below equation.
[Equation 1]            P      b        ≈          Q      ⁡              (                  γ                )              =      Q    ⁡          (                                    W            ·                          h              ⁡                              (                t                )                                      ·                          P              ⁡                              (                t                )                                                                        R              ⁡                              (                t                )                                      ·                          I              ⁡                              (                t                )                                                        )      
On the condition that       Q    ⁡          (      x      )        =            1                        2          ⁢          π                      ⁢                  ∫        x        ∞            ⁢                        exp          ⁡                      (                                          -                                  y                  2                                            /              2                        )                          ⁢                                  ⁢                  ⅆ          y                    and γ is energy per bit to interference plus noise density ratio (EINR), W is bandwidth of a channel, h(t) is path loss, P(t) is transmitted power of transmission, I(t) is interference transmitted power plus background noise transmitted power, and R(t) is data rate. It is possible that h(t) is changed due to moving of a mobile terminal, and R(t) or I(t) is changed due to changing of transmission rate in actual system. However, in the CDMA systems, power control is performed so that the BER of the received data is fixed by adjusting P(t). As shown at the equation 1, γ and Pb is not the function for time(t).
When the video information is sent in the conventional CDMA systems, if the services are the same kind of services such as video communication network telephone and conference, and synchronized video services, target BER of all the packets in the video sequence is chosen as the same value. To satisfy the mentioned target BER, power control is applied. So, with the same channel condition, the BER in the each packet becomes the same as all the video packet of the same kind of services.
As mentioned above, only with the same kind of services, the conventional system controls the target BER for all the packets in the video sequence and controlling transmitted power to maintain this target and performs the FEC to keep the value. But when the video data is transmitted to a channel on which noise exists, controlling the same target BER for all the video packets is not efficient due to the reason as follow. In case of that the packet is damaged during the video information is transmitted split in packet unit, the influence on the image quality is different depending on the characteristic of the video data which is stored in each packet. For example, even though a packet is damaged by channel error, if the packet includes the video information, which can be error concealed well at the decoder, the packet hardly influences on the image quality. On the contrary, if the packet, which includes the video information, which can not be error concealed well at the decoder, is damaged, a big distortion occurs on the video decoder and thus, this packet is more important compared to other packets. So, it is not efficient that controlling the same target BER for all the packets which support same kinds of services and keep that.
Also, in the conventional system, the target BER of each packet is controlled without concerning the number of the wireless channel through which the video packet passes. In other words, during communication, the packet passes through the wireless channel twice between two wireless ends, once between wireless end and wired end, and none between the wired ends. And in the conventional system, the same target BER is controlled without classifying the mentioned differences. But this method may provide too large target BER between two ends for a packet which passes many times through the wireless channel. And it may provide too small target BER between two ends for a packet which rarely passes or not through the wireless channel. Therefore, it is not desirable that controlling the same target BER in each channel without concerning the numbers of wireless channels which the video packet passes through.